rbd: remove rbd_parent_request_{create,destroy}()

rbd_parent_request_create() takes a ref on obj_req for child_img_req.
There is no point in doing that because child_img_req is created on
behalf of obj_req -- obj_req is the initiator and can't be completed
before child_img_req.

Open-code the rest of rbd_parent_request_create() and remove it along
with rbd_parent_request_destroy().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>